The OFDMA downlink of LTE (long term evolution) mobile wireless data systems operated by carriers in the licensed bands has always been hindered by key system level bottlenecks and congestion. Also, WiFi has been solely deployed in unlicensed bands where huge spectrum is available in 2.4 Ghz (UHF) and 5 GHz band. Up until now, WiFi has been the only major player in these bands. But as the licensed bands are hitting congestion, operators are looking to deploy LTE in unlicensed spectrum as well. There are two main types of LTE in unlicensed band, 1) LTE-U (Qualcomm) and LAA (3GPP). With these new technologies, incumbent WiFi has no choice but to share spectrum resources in TDD (time division duplex) with LTE-U and LAA. Hence, spectrally efficient utilization of the unlicensed bands is becoming more critical for WiFi downlinks. Similarly, LTE on the unlicensed bands has to cooperate with incumbent WiFi so that new technologies offering enhanced downlink efficiency for any or all of LTE, LTE-U, LAA, and WiFi systems are needed. Other higher bandwidth OFDMA technologies such as “GigaFi” are being introduced to provide higher data rates than can currently be achieved with WiFi. New technologies offering enhanced downlink efficiency for GigaFi or other types of next-generation enhanced short range or long range systems are also needed.
LTE and WiFi systems typically involve a multi-user downlink that employs orthogonal frequency division multiplexed multiple access (OFDMA). For example, in LTE systems, different mobile units will be assigned respective resource blocks that each identify a set of 12 OFDM tones (tone=line frequency). Each resource block includes 12 15 KHz wide sub-carriers (on the frequency axis) and 14 (or 12) OFDM symbols on the time axis. Hence each resource block will carry 168 (or 144) data or control symbols at a specified bit loading that can consist of QPSK (4-QAM), 16-QAM, 64-QAM, or 256-QAM. In LTE, each resource block will be transmitted in 1 ms and occupies 180 KHz bandwidth. Other technologies such as cable modems use a different downlink channel other than OFDMA, but have similar issues with different attached cable modems having different quality downlink channels.
Consider a simplified exemplary system where half of the mobile units have strong channels and are assigned resource blocks that use 256-QAM (eight bits per data tone) while the second half of the mobile units have weak channels and are assigned resource blocks that use 4-QAM (two bits per data tone). In such an example, due to the lower bit loading used in the downlink resource blocks assigned to the second half of the mobile units, the total downlink throughput is much less than the maximum that could be achieved if all of the mobile units had strong channels that could support resource blocks that used 256-QAM. It would be desirable to be able to increase the net throughput of the LTE and WiFi downlinks by allowing the data symbols of resource blocks associated with the weak channels to carry additional bits that could be received and decoded by mobile units that can see those same data tones through the lens of the strong channels.
Tiled-building-block trellis codes are a family of codes that typically use trellis codes such as convolutional codes and turbo codes (e.g., parallel concatenated convolutional codes) as distinct codes in a multilevel coded system. Such coded systems are described in detail in U.S. Pat. No. 8,077,290 and M. A. Nairn, J. P. Fonseka, And E. M. Dowling, “A Building-Block Approach for Designing Multilevel Coded Systems,” IEEE Corn Letters, Vol. 19, NO. 1, January 2015, “the Naim reference.” U.S. Pat. No. 8,077,290 is incorporated herein by reference and the reader is referred to this patent to better understand the background of tiled-building-block trellis codes. A building-block-trellis code is used to construct a small compact signal constellation building block which is called a “coded-constellation-building block,” or, a “building block” for short. A tiling code is also employed to allow the small powerful building blocks to be tiled to form larger constellations with a tile spacing that is selected to preserve the building block's MSED. Associated with the tiling code is a signal constellation called the “tiling constellation.” At each constellation point of the tiling constellation is placed a copy of the building block. Each constellation point of the tiling code is referred to as a “tiling point.” The “intra-block MSED” is defined as the MSED between constellation points within a building block, and the “tiling MSED” is defined as the MSED between the centers of the tiled building blocks, i.e., the MSED between tiling points in the tiling constellation. In terms of coded sequences, the “sequence-level intra-block MSED” is the distance between coded sequences of constellation points within a building block and the “sequence-level tiling MSED” is the distance between coded sequences of tiling points (tile center locations).
It would be desirable to have methods, apparatus and systems for improving a systems-level data rate on a communications link such as those used in the downlink of OFDMA systems. It would be desirable to have a technology that uses tiled-building-block encoding/decoding in a configuration that allows different receivers coupled with different parallel downlink channels with different channel qualities to decode different received versions of the transmitted signal constellation at different levels of resolution. It would be desirable to have a technology that could reduce or eliminate existing inefficiencies in the downlink of OFDMA systems to thereby allow the downlink to operate with a significantly higher data rate, thus significantly increasing system level throughput and spectral efficiency.